Professional social networking service LinkedIn today said it is investigating reports that hackers broke into its systems and accessed the usernames and hashed passwords of the social network's 6.5 million members.

The data was said to be posted on an online Russian hacker forum.

In numerous Twitter messages, LinkedIn told its members that it's investigating the breach reports, and that it can't yet confirm that hacker had accessed the site.

One said: "Our team is currently looking into reports of stolen passwords. Stay tuned for more."

One security researcher today said that he has downloaded a file from a Russian hacker website containing more than 6.4 million hashed passwords.

Marcus Carey, a security researcher at Rapid7, said he also downloaded two separate files containing more than 300,000 passwords collected by the hackers. The hackers accessed the passwords by using simple password cracking tools, Carey said.

Though it is not immediately possible to confirm if the hashed passwords were in fact accessed from LinkedIn's servers, there are numerous anecdotal reports that users have seen their LinkedIn password posted online, he said.

So far, he added, there is no evidence that emails associated with the passwords have also been accessed, though that remains a possibility.

Carey noted that the hackers might still have access to the LinkedIn servers.

According to him, a look at the data that was posted online suggests that the hackers may have had access to the data for sometime.

Users of LinkedIn should immediately change their passwords to protect their accounts, he said.

After reviewing the data posted online by the apparent hackers, Carey said, "We don't know who is behind this but [they] definitely had access to the LinkedIn database for at least the last week."

"I want to emphasize is that we don't know if the attackers still have access to the LinkedIn system," he added.

According to Carey, a manual inspection of the leaked passwords show that the site was protected by using the Secure Hashing Algorithm-1 (SHA-1) format.

Because the protection offered by SHA-1 isn't foolproof, security experts have for some time now recommended that organizations use a technique called salted hashing to protect sensitive data, like passwords.

The fact that the passwords have been hashed using straight SHA-1 makes them somewhat easy to crack using brute force methods, he said.

Many of the 300,000 or so passwords that have already been posted in clear text online were cracked using a password cracking tool called John the Ripper, Carey said.

Users should immediately change their passwords, and keep a close eye on updates on the incident from LinkedIn, he said.

If it turns out that the hackers still have access to the database, LinkedIn users may need to change their passwords again, he said.

"The silver lining here is that we know the attack took place, so users can at least change their passwords," he said.

The real danger comes when such attacks aren't discovered until long after they are launched. The fact that the attackers don't appear to have the matching email addresses for the stolen passwords is also a good sign fro LinkedIn members, he said.